Packet Radio Services offers packet switched communications over radio links in e.g. GPRS and UMTS. Data is disassembled and transmitted in packets or Protocol Data Units (PDUs). Upon reception, the PDUs are reassembled.
FIG. 1 illustrates protocol layers for GERAN (GSM-EDGE Radio Access Network) A/Gb mode and will be explained in some detail below. All functions related to transfer of Network layer Protocol Data Units, N-PDUs, shall be carried out in a transparent way by the GPRS network entities.
3rd Generation Partnership Project (3GPP): Technical Specification Group GERAN, Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (Release 4), 3GPP TS 43.064 V4.3.0, France, February 2002, provides the overall description for lower-layer functions of GPRS and EGPRS (Enhanced GPRS) radio interface, Um. In the sequel GPRS refers to both GPRS and EGPRS in not explicitly stated otherwise. An EGPRS mobile/base station is a GPRS compliant mobile/base station with additional capabilities for enhanced radio access protocol features and enhanced modulation and coding schemes. The support of EGPRS is optional for mobile station and network.
3rd Generation Partnership Project (3GPP): Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol (Release 4), 3GPP TS 44.060 V4.8.0, France, September 2002, specifies the procedures used at the radio interface for the General Packet Radio Service, GPRS, Medium Access Control/Radio Link Control, MAC/RLC, layer. The RLC/MAC function supports two modes of operation:                unacknowledged operation; and        acknowledged operation.        
Section 9.3 describes operation during RLC data block transfer. RLC acknowledged mode, RLC-AM, operation uses retransmission of RLC data blocks to achieve high reliability. RLC unacknowledged mode, RLC-UM, operation does not utilize retransmission of RLC data blocks.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, Physical Layer Procedures, 3G TS 25.322 v3.5.0, France, December 2000, specifies three data transfer services of radio link control, RLC:                transparent data transfer service,        unacknowledged data transfer service, and        acknowledged data transfer Service        
Subsections 4.2.1.1 and 4.2.1.2 describe transparent mode entities and unacknowledged mode entities. One difference of the two modes resides in management of packet overhead.
In transparent mode no overhead is added or removed by RLC. In subsection 4.2.1.3 an acknowledged mode entity, AM-entity, is described (see FIG. 4.4 of the 3GPP Technical Specification). In acknowledged mode automatic repeat request, ARQ, is used. The RLC sub-layer provides ARQ functionality closely coupled with the radio transmission technique used.
3rd Generation Partnership Project (3GPP): Technical Specification Group Core Network; Digital cellular telecommunications system (Phase 2+); Mobile Station (MS)—Serving GPRS Support Node (SGSN); Subnetwork Dependent Convergence Protocol (SNDCP) (Release 5), 3G TS 44.065 v5.1.0, France, September 2003, provides a description of the Subnetwork Dependent Convergence Protocol, SNDCP, for GPRS. SNDCP entity performs multiplexing of data coming from different sources to be sent using service provided by the LLC (Logical Link Control) layer,
3rd Generation Partnership Project (3GPP): Technical Specification Group Core Network; General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface (Release 5), 3GPP TS 29.060 V5.8.0, France, December 2003, defines the second version of GTP used on:                the Gn and Gp interfaces of the GPRS;        the Iu, Gn and Gp interfaces of the UMTS.        
Within GPRS (and UMTS) Gn interface is an interface between GPRS Support Nodes (GSNs) within a PLMN and Gp interface is an interface between GPRS Support Nodes (GSNs) of different PLMNs. In UMTS Iu interface is an interface between RNC and Core Network.
A Gb interface is an interface between an SGSN (Serving GPRS Support Node) and a BSC (Base Station Controller). An A interface is an interface between BSC and MSC (Mobile Services Switching Center).
GPRS Tunneling Protocol, GTP, is the protocol between GPRS Support Nodes, GSNs, in the UMTS/GPRS backbone network. GTP allows multi-protocol packets to be tunneled through the UMTS/GPRS Backbone between GSNs and between SGSN (Serving GSN) and UTRAN (Universal Terrestrial Radio Access Network).
3rd Generation Partnership Project (3GPP): Technical Specification Group Core Network; Mobile Station—Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) layer specification; (Release 4), 3GPP TS 44.064 V4.3.0, France, March 2002, defines the Logical Link Control, LLC, layer protocol to be used for packet data transfer between the Mobile Station, MS, and Serving GPRS Support Node, SGSN. LLC spans from the MS to the SGSN. LLC is intended for use with both acknowledged and unacknowledged data transfer.
LLC supports two modes of operation:                Unacknowledged peer-to-peer operation, LLC-UM, and        Acknowledged peer-to-peer operation, LLC-AM.        
In unacknowledged operation logical link entity may initiate transmissions to a peer entity without prior establishment of a logical connection with the peer entity. LLC does not guarantee in-order delivery. LLC can detect errors in a received frame, and, depending on whether the frame is sent in protected mode or not, either discard or deliver the erroneous frame. No error recovery procedures are defined at the LLC layer. Higher-layer protocols can be used to provide reliability, if needed. This mode of operation is known as Asynchronous Disconnected Mode, ADM.
With acknowledged operation a balanced data link involves two participating entities, and each entity assumes responsibility for the organization of its data flow and for error recovery procedures associated with the transmissions that it originates. Each entity operates as both a data source and data sink in a balanced link, allowing information to flow in both directions. This mode of operation is known as Asynchronous Balanced Mode, ABM, and provides a reliable service with in-order delivery.
European Patent Application EP1318691 describes a method for informing the SGSN about a mobile station cell-change operation in the GPRS.
International Patent Application WO03032672 discloses a method of optimization of handover procedures in GPRS comprising the old SGSN sending identification response directly to the new SGSN.
International Patent Application WO0079808 claims a method of reducing delay time for a mobile station being handed over from an old SGSN to a new SGSN during a call handling a real-time payload in a GPRS packet switched radio telecommunications network comprising shortening the inter-SGSN Routing Area Update interruption interval and implementing low latency requirements and shaping of packet traffic.
International Patent Application WO02085048 describes a handover procedure for use in a GPRS network, reducing the need for re-sequencing in SGSN. Old SGSN sends a message to GGSN (Gateway GSN) requesting data transmission to stop. Data at old SGSN, for transmission to MS, is transferred to new SGSN and transmission from GGSN is resumed when handover is complete. GGSN then transmits data to new SGSN.
In U.S. Patent Application US20010019544 the GGSN and SGSN are allowed to finish up on-going transactions before moving the context to the new SGSN. The first (old) SGSN is operating as a temporary anchor in response to an inter-SGSN routing area update.
European Patent Application EP1345463 reveals buffering of TCP packets in a mobile node during handover.
FIG. 2 illustrates schematically some network elements involved in packet switched handover. A source SGSN source SGSN connected to a gateway GSN GGSN supports data traffic to a mobile station MS via a source base station subsystem source BSS. A base station change may be initiated, e.g. as the mobile station moves, towards a base station of a target base station subsystem target BSS supported by a target SGSN target SGSN.
In prior art lossy type of packet switched handover is used for services requiring short delay but allowing some data loss at cell change, e.g. speech services. For lossy handover downlink data is typically duplicated by the source SGSN and sent both to the source BSS for further transmission to the mobile station in the current cell, and to the target SGSN target SGSN.
The target side (BSS/SGSN) can either discard the forwarded data until the MS has indicated its presence in the target cell or, blindly, send the data without information available on whether or not the MS is present in the target cell. In case of blindly sending the data, the mobile station has been ordered to perform the handover and has synchronized towards the target cell, the downlink data flow is already ongoing and the mobile station can immediately start the uplink data flow. No acknowledgement of received data is required, neither in uplink nor in downlink.
According to prior art solutions, data losses will occur, e.g., when data packets sent to source BSS from source SGSN are discarded in source BSS when a mobile station is handed over from source BSS to target BSS. Losses will also occur if e.g. packet data forwarded to MS via the target SGSN and target BSS experiences a delay that is less than the delay associated with the MS processing the handover command and acquiring synchronization.
Lossless type of packet switched handover, PS handover, is used for services that are sensitive to data losses but can accept a certain delay. The typical characteristics of a lossless handover are currently based on acknowledged RLC and LLC protocols and the SNDCP protocol operating in acknowledged mode. During the PS handover the downlink data flow is forwarded from the source SGSN to the target SGSN. The target SGSN buffers the downlink data until the mobile station has indicated its presence in the target cell. The SNDCP layer at both the MS and SGSN assigns N-PDU Send number to each N-PDU sent and maintains N-PDU Receive number for each received N-PDU for any given bi-directional packet service. When a handover is performed of such a service the number of the next expected uplink and downlink N-PDU is exchanged between the MS and the SGSN in handover signaling messages allowing precise knowledge of where packet data transmission should resume after handover.
None of the cited documents above discloses lossless packet switched base station handover or radio cell change in. LLC unacknowledged mode, LLC-UM.